Various types of intrusion detection systems have been proposed, including systems based on ultrasonics, optical detection, infrared detection, and microwave detection. In one common type of system, energy is transmitted into a selected space surrounding the area to be protected, and energy reflected from objects in such space is received and monitored. An alarm is activated if reflections are received indicating the presence of an intruder.
One class of prior intrusion detection systems has relied on the doppler principle. In these systems, microwave energy reflected from the selected space is examined for the presence of doppler shifted components, indicating the presence of a moving target within the space.
Doppler intrusion detection systems to date have not proven adequately sensitive to slow moving targets, and have been inordinately sensitive to small animals and environmental factors such as rain, wind, noise, and the rate of change of these factors. One approach to minimizing these adverse effects has been to transmit RF energy into the monitored space from buried leaky coaxial cables. Such systems have suffered from the fact that the cables were buried and thus a majority of the RF energy was kept below ground, and further from the fact that a pole vaulter could cross the detection zone without triggering an alarm. Systems based on buried cables also typically include large returns from outside the desired detection zone, and the systems are subject to additional degradation by high conductivity soils.